1. Field of the Invention
This invention relates to an FM demodulator and, more particularly, to an FM demodulator of the kind in which a VCO (voltage-controlled oscillator) employing a varactor diode is used as a demodulator.
2. Description of the Prior Art
Conventionally, various types of circuit are used as FM demodulators. For example, an FM demodulator based on the PLL (phase locked loop) method using a varactor diode has a construction as shown in FIG. 1, in which a frequency-modulated (FM) signal is input from an input portion IN to a phase comparator 11 and an oscillation frequency signal from a VCO 12 is also input from another input portion 13 to the phase comparator 11, respectively. Then, an error signal based on the phase difference between the above FM signal and the oscillation frequency signal is output from the phase comparator 11. This error signal is input as an oscillation frequency control signal to the VCO 12, thereby controlling the oscillation frequency of the VCO to follow the frequency deviation. As is obvious from the above-mentioned construction, the signalling system constitutes a PLL and a signal on the output line of an amplifier 4 is output (OUT) as a demodulation signal.
The VCO 12 includes a varactor diode and the oscillation frequency is controlled by controlling the voltage to be applied to this varactor.
However, in the above-described conventional FM demoduator, since the change of the electrostatic capacity with respect to the change of the voltage applied to the varactor is not ideal, this causes a deviation of the output frequency of the VCO 12 (which changes due to the change of the electrostatic capacity of the varactor) with respect to the ideal output frequency, so that there is a drawback in that this deviation is output as a distortion component of the demodulation signal on the output line of the amplifier 4 and thus the sound quality suffers.